1. Field of the Invention
The present invention relates to a lens apparatus equipped with a diaphragm mechanism, and an image capturing apparatus including the lens apparatus, which is suitable for, for example, an interchangeable-lens digital single-lens reflex camera, a digital still camera, a digital video camera, and an interchangeable-lens digital video camera.
2. Description of the Related Art
A diaphragm mechanism (a diaphragm unit) serves to change the size of a diaphragm aperture formed by a plurality of diaphragm blades, and to increase or decrease the amount of light passing through the diaphragm aperture by moving the plurality of diaphragm blades in opening and closing directions using an actuator such as a stepping motor.
Recently, in an interchangeable-lens digital single-lens reflex camera, the number of continuous shots in still image shooting has been one of the barometers for evaluating the performance as a camera system and it becomes important to cope with high-speed operation of a diaphragm mechanism. Here, when a driving force of an actuator is transmitted to diaphragm blades and the diaphragm is opened or closed, the variation in diaphragm accuracy or hysteresis in driving conversion occurs by the influence from mechanical backlash between the parts in the diaphragm mechanism or the variation in step accuracy of a stepping motor.
The reason is because, if the mechanical backlash is completely removed, when a sliding resistance is increased due to overlapping of a plurality of diaphragm blades, bending of the diaphragm blades, or thermal contraction of the components, the risk of malfunction occurs.
A following countermeasure is taken as control of a diaphragm mechanism focusing on the diaphragm accuracy rather than the number of continuous shots. The diaphragm mechanism is controlled to have each target aperture value by, after aperture opening operation in which an aperture diameter is a fixed diameter, measuring light in the open state and performing driving in the same direction from the open side to a small aperture side. Accordingly, in the operation of the diaphragm mechanism to the target aperture values, it is required to perform the aperture opening operation on the diaphragm mechanism each time and, there is a limit in coping with a high-speed operation of the diaphragm mechanism.
For example, as a measure of improving diaphragm accuracy, Japanese Patent Application Laid-Open No. 11-160754 discusses that swing of diaphragm blades during diaphragm operation is prevented by disposing a pressing spring for each diaphragm blade and maintaining pressure on the cam followers formed at the blades with the springs against the cam.
Further, Japanese Patent Application Laid-Open No. 2007-199578 discusses that the position difference in diaphragm accuracy is improved by pressing a windmill to a restricting portion that restricts the rotational center of the windmill formed on a diaphragm ground plate.
Accordingly, diaphragm accuracy or hysteresis in driving reversal may be reduced.
However, in the related art discussed in the patent applications described above, the diaphragm accuracy and the position difference are improved, while operational load is increased when the diaphragm blades are opened and closed. Accordingly, it is remarkably difficult to cope with high-speed operation of the diaphragm mechanism for securing the number of continuous shots.